Process of treating black liquor produced in the manufacture of sulphate pulp



Get. 9, 1923.

G A. RICHTER ET AL I E MANUFACTURE PROCESS OF TREATING. BLACK LIQUOR PRODUCED IN TH OF SULPHATE PULP Original Filed Nov. 4 1920 Patented oer. a, was.

new snares GEORGE A. RICHTER AND DOUGLAS H. MQMURTRIE, F BERLIN, NEW HAMPSHIRE, AS-

SIGNORS TO BROWN COMBANY, OF BERLIN, HAMPSHIRE, A CORPORATION OF MAINE.

PROCESS OF TREATING BLACK LIQUOR PRODUCED IN THE MANUFACTURE OF SUL- PHATE PULP.

Application filed November 4, 1920, Serial N 0. 421,769. Renewed February 9, 1923.

To all whom it may concern.-

Be it known that we, GnoRGE'A. Rrorrrnn and DOUGLAS H. MCMURTRIE, citizens of the United States, residing at Berlin, in the county of Coos and State of New Hampshire, have invented new and useful Improvements in Processes of Treating Black Liquor Produced in the Manufacture of Sulphate Pulp, of which the following is a specification.

The present invention relates generally to the manufacture of chemical wood pulp by the sulphate process, and more especially to the preparation of the cooking liquor employ-ed for digesting the wood.

The name sulphate pulp or sulphate process ascribed thereto arises from the fact that heretofore it has been the practice to employ sodium sulphate for the purpose of producing sodium sulphide, by reduction, in W the smelting o eration which forms a part of the process ollowed in the preparation of the cooking liquor.

In the ordinary method of preparing the cooking liquor, which includes the recovery of the soda content of the black liquor, resulting from a previous digestion or cooking operation after it has been separated from the pulp, it has been customary by evaporation to concentrate the. black liquor to a more or' less viscous condition, and then to add there to, either while the liquor is in a liquid form or after it has been more or less charred, a certain proportion of sodium sulphate, and smelt the mixture in a smelting furnace; By this smelting of the mixture of the sodium carbonate, sodium sulphate, the organic matter contained in the black liquor, and the remaining soda content of such .li uor, the

sodium sulphate is reduced to so ium Sill-1 phide in a reducing atmosphere, and the melt or molten material, flowing from the furnace, consists principally .of sodium sulphide and sodium carbonate, with more or less sodium h droxide and unreduced sodium sulphate. This melt is received in a bath of Water, preferably more or less alkaline, and forms a solution. Subsequently lime is added thereto in the causticizingitanks to convert the sodium carbonate to sodium hy- W droxide, and the preci itated calcium carbonate is removed by fi tration or otherwise.

7 The filtrate or remaining solution, which forms the liquor for digesting the wood in able to fix the same as sodium sulphide, and

thus to increase to a predetermined extent the proportion of sodium sulphide in the efliuent from the smelting furnace.

For satisfactory results, the cooking liquor should comprise both caustic soda and sodium sulphide, in about the proportions of 7 to 3 based in terms of total alkali. Hence it is customary, in the usual recovery processes, to make up losses in total amount of alkali by the addition of sodium sul hate, which is converted to sodium sulphi e and sodium carbonate in the smelting operation. Depending upon the reduction, in the furnace, variable results are secured, and the eflluent varies in the ratio of sodium carbonate to sodium sulphide.

By our process, which is hereinafter described, we secure a direct control, by introducing sulphur separatel and not fixed soley as sodium sulphate. n'addition, we may also introduce sodium carbonate into the mixture to be smelted, or during the 'causticizing operation of the recgvered melt, thereby securing indepedent control of each constituent,

These two materials Na,CO and S, may be used together with, or as a substitute for, the

usual salt cake, Na,S;O,. In other words, we

may eliminate the addition of sodium sulphate, in the recovery process, or may employ therewith free sulphur, and, if necessary, sodium carbonate. Thus the free sulphur may be substituted directly for the combined sulphur contained in the salt cake, or may be added to increase the proportion of total sulphur to total sodium content, thereby increasing the proportion of sodium sulphide to sodium carbonate in the melt leaving the smelting furnace.

F or example, with concentrated spent minuted form, to produce a suitable quantity of sodium sulphide in the cooking liqnor. lin ordinary practice, the addition of sulphur in the proportion'of 1% to 3% by weight of concentrated black liquor produces satisfactory results. The mixture may be exploded in a combined boiler and smelting furnace, by the process and apparatus set forth inlU. S. Letters Patent No. 1,326,414, granted December 30, 1919, to Hugh K. Moore and John T. Quinn. If desired, however, the sulphur in solid or molten form may be introduced into the black ash resulting from the disk-evaporation and the charringin a rotary kiln or drum ofthe spent. black liquor, or it may be introduced into the smelting furnace for admixture therein with the inorganic content of the black liquor.

The fixation of the sulphur as sodium sulphide, by the alkaline content of the black liquor, may result in a deficiency in sodium carbonate in the melt, and this deficiency may be made up by adding a suitable amount of sodium carbonate to the liquor before the causticizing, operation. For example, weadd an amount of sodium carbonate as determined by proportion from the chemically equivalent weights of sodium carbonateand sulphur,namely 106 to 32.

The sodium carbonate, however, may be mixed with the concentrated black liquor or introduced directly into the smelting furnaee Again, free sulphur may be introduced into the black liquor together with a suit able quantity of sodium sulphate. For example, we have secured excellent. results,

'by the addition of from to pounds of comminuted sulphur per ton of produced pulp, in which about 400 pounds of sodium sulphate per ton of pulp were used. In this case there was maintained in the cooking liquor a desirable high sulphidity.

The efiuent from the smelting furnace is received as ordinarily in water or an alkaline bath, and the solution, after being causticized, and decanted or filtered, is employed for the digestion of wood in the manufacture of sulphate pulp. The spent black liquor, resulting from the di estion, is then evaporated to concentrate form, and the herein described cycle of operations is repeated.

0n the accompanying drawing we have shown more or less conventionally and diagrammatically, a bda recovery system substantiall as illustrated in the Letters Patent hereinbefore referred to by which we have carried out our process. in this particular system which has been selected only for purposes of illustrating one form which may be used, but to which we are not neeaeeo limited, the concentrated black liquor is delivered from a storage tank 1 into a mixing vat 2 into which powdered free sulphur is delivered as desired from a spout 3. Sodium sulphate and sodium carbonate may be delivered as required into the mixing vat from spouts at and 5 respectively. The mixture is pumped by a pump 6 under high pressure through a circulating system comprising the conduits 7 8, 9, the pump 10, coil 11, manifold 12, and check valve 13. The

-coil 11 is within a. heater 14 by which the mixture is heated to a high temperature so as to generate an explosiveforce therein, the pressure on such mixture being greater than the vapor pressure. From the manifold 12, nozzles lead into the front-of; the furnace, so asto discharge the mixture into the upper portion of the combustion chamber. For supporting the combustion and,

producing an oxidizing atmosphere in the upper part of the chamber, air pipes 15 or tuyeres extend into the furnace for delivering air thereto, under pressure if desired.- A reducing atmosphere of carbon monoxide is created in the lower portion of the combustion chamber near the sloping floor wherein the molten soda flows forwardly and is delivered through one or more spouts 16 into a mixing vat or tank 17. i

While we do not desire to be committed to any theory of operation or reaction, we have reason to believe that the first reaction, involving the sulphur sprayed with the black liquor into the furnace, is the oxidation of the sulphur to S0, and that thereupon the s+ Na C'Q, so, ri se, co,

inasmuch as there is an oxidizing atmosphere in the upper part of the furnace where the mixture is delivered and exploded, it may, perhaps, be fairly assumed that part of the S 0, is converted to sodium sulphate (Na SOQ, due to the further oxidation'of sodium 'sulphite (Na SOQ). The sodium sulphite and sodium sulphate reaching the floor of the furnace, are reduced to sodium sulphide.

' It is alsopossible, and indeed probable, that since the concentrated black liquor contains caustic soda, part of the sulphur may react directly therewith, forming sodium sulphide and sulph-hydrate, but if so, doubtless this side reaction involves only a small part of the sulphur. If portions of the sulphur are not burned in the upper part of the combustion chamber and reach aeeaeeo out to form sodium sulphide. We regard it as highly desirable in the use of a furnace as herem described that the sulphur should enter an oxidizing atmosphere first, and that there should also be a reducing 'zone in the furnace, in order that any sodium sulphite or sodium sulphate formed should be reduced to the sulphide.

What we claim is 1. A process comprising burning and mnelting spent black liquor resulting from the digestion of wood in the manufacture of sulphate pulp, in-the presence of free sul-- quantities to compensate for the loss thereof l by the chemical reaction of sulphur with sodium carbonate initially in the spent liquor. I

4. The herein described process which comprises concentrating black liquor contaimng residual sodium carbonate, mixing free sulphur and sodium sulphate therewith, smelting the mixture, and recovering the molten efiuent with a predeterminm percentage ofsodium sulphide.

5. The herein described process which comprises concentrating black liquor containlng residual sodium carbonate, mixing free sulphur and sodium. carbonate there with, smelti the mixture, and recovering the molten e uent.

6. The herein described process which comprises concentrating black liquor containlng residual sodium carbonate, mixing free sulphur, sodium carbonate and sodium sulphate therewith, smelting the mixture, and recovering the molten efiuent.

In testimony whereof we have afixed our signatures.

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